How to assign separate material to separate mesh
Actually your objects are linked-duplications, it doesn't look like because you gave them different booleans. So any modification on one of them (material or mesh) will be replicated on the other. If you want to keep their mesh linked but want to have separate materials, choose Object instead of Data in the Material panel:

If you want to completely separate them, go in the Object menu &gt; Relations &gt; Make Single User &gt; Object &amp; Data.